The powder molding method has recently been used widely for applications typified by automotive interior materials, adhesives for interlining, etc. because of its advantages such as being capable of easily molding products with complicated shapes (undercut, deep drawing, etc.), being capable of making uniform the thickness of products, and being capable of affording high yields of materials.
Although a soft polyvinyl chloride powder has mainly been used for the powder molding method, the plasticizer contained migrates to the surface and impairs the soft feeling if such a powder is used for long term under some environment of use. When an automobile is disposed and then subjected to incineration treatment, hydrogen chloride gas is generated in accordance with the incineration temperature, so that the incineration furnace may be corroded in some cases. For this reason, polyurethane resins have also been used in recent years. While conventional polyurethane resins are high in cost and have environmental problems because they are synthesized in organic solvents, a method for producing a spherical polyurethane resin powder in an aqueous medium has been proposed (see, for example, Patent Document 1).
However, since the polyurethane resin powder produced by the method of Patent Document 1, which is a material excellent as a powder slush material for automotive interior parts, uses a ketimine compound as an extender, such a powder has room for improvement in low odor property relating to vehicle compartment comfortableness, which has recently been increasingly demanded.
In the method of Patent Document 1, since an emulsifier is included in water as a dispersion medium and dispersion is performed with a high shear disperser when dispersing polyurethane resin powder forming components in water, fine particles of several μm and coarse particles of several hundred μm are formed in a small amount. If the resin powder obtained from this dispersion is used directly for a slush molding application, powder flowability deteriorates and pinholes, etc. are formed in the surface of a molded article. For this reason, the resin powder is required to be classified into particle size distribution suitable for the slush molding application.
A material for slush molding which has been colored in order to exhibit high quality feeling is in use as a skin material of an automotive interior component. The material has been colored by synthesizing uncolored particles, and then dusting a colorant such as an inorganic pigment or an organic pigment onto the surfaces of the uncolored particles. If this coloring method is performed using a spherical polyurethane resin powder, however, by shear at the time of stirring and mixing in the step of the coloring or in a step subsequent thereto, pigment particles which adhere onto the resin particle surfaces are slipped down from the surfaces, or the pigment particles aggregate with each other on the particle surfaces. Thus, this method has a problem that the original color of the coloring agent is not expressed and a lump of the pigment is intermingled with the resultant product.
In order to solve such a problem, a method has been proposed in which a colorant is mixed at a stage where the resin is still only a liquid prepolymer before being granulated, and subsequently the mixture is granulated (Patent Document 2). In this method, since pigment particles have been trapped within resin particles, the pigment fails to slip down from resin particles or aggregation of pigment particles on particle surfaces does not occur. As compared with the above-described method of coloring particle surfaces, however, this method is poorer in productivity because it is unavoidable to wash production facilities carefully whenever the color of products is changed.